It has been known in the past to provide fuel or air supply cut-off devices for engines. For instance during the drilling of an oil or gas well several operations are performed which may result in natural gas flowing into the enclosure around the operating engines and attaining high concentrations therein. Such natural gas could enter diesel engines through the air intake and, even though the fuel oil is shut off, the engine could continue to operate. This would create hazards such as sparks being emitted from the engine exhaust which could ignite the gas and air mixture around the engines and also the generous supply of fuel could cause the engine to attain excessive speeds which could be damaging to the engine itself. There have been prior attempts at providing engine air cut-off devices, for example, reference can be made to Canadian Patents Nos. 587,457 (Oakes, Nov. 24, 1959); 788,333 (Gordon, June 25, 1968); or 914,644 (Verheul, Nov. 14, 1972). The cut-off devices of these patents are probably effective in their own right but each involves a structure which is large and unwieldy and which necessitates a relatively complex actuation structure. In many instances external actuation via a hydraulic or pneumatic cylinder is utilized or, alternatively, a spring biased type of actuation may be utilized.